1. Field of the Invention
The present invention relates to a clamping apparatus for adjusting the DC level of an analog signal to a predetermined value in analog-to-digital signal conversion, and also to a gain control apparatus for adjusting the amplitude of an analog signal to a predetermined value. Both apparatuses are adapted for use particularly in digitizing a video signal.
2. Description of the Prior Art
In broadcasting equipment as well as consumer-use audio and video equipment inclusive of television receivers and video tape recorders, it is generally usual of late to adopt the technique of converting an analog signal to a digital signal and thereby processing the signal in a digital form. When an analog signal is digitized, the DC level or the amplitude of such analog signal inputted to an analog-digital (A-D) converter needs to be adjusted to a predetermined value. For this purpose, it is customary to employ a clamping apparatus or a gain control apparatus with a feedback loop which detects the DC level or the amplitude of the signal digitized by means of an A-D converter and controls the analog signal inputted to the A-D converter.
In the conventional clamping apparatus contrived for processing a video signal, there are known some typical ones which will be described below.
An exemplary prior technique is disclosed in Japanese Patent Application Laid-open No. 63-16476 (1988) and U.S. Pat. No. 4,562,471, wherein the DC level of a digitized video signal is maintained at a predetermined value by first detecting the DC level error from such video signal, then integrating the error in the form of a digital signal to obtain a digital DC signal, subsequently converting such digital DC signal to an analog DC signal by means of a digital-analog (D-A) converter, and adding the converted signal to the analog video signal inputted to the A-D converter.
However, such conventional method has a disadvantage of necessitating a high precision D-A converter.
As for the clamping apparatus of another type where no D-A converter is required, a prior art example is disclosed in Japanese Patent Application Laid-open No. 60-16769 (1985), wherein the DC level of a signal is maintained at a predetermined value by first pulse-width modulating the digital DC signal obtained similarly to the above-described example, thereby producing a digital pulse signal of a duty ratio corresponding to the amplitude of the digital DC signal, then converting the modulated signal to an analog DC signal by means of an analog integrating circuit, and adding such converted signal to the analog video signal inputted to the A-D converter as in the foregoing example.
However, since the analog DC signal is obtained by pulse width modulation and analog integration in the above method, there exists a fault that undesired DC level variations based on the ripple are generated if a small time constant is selected for analog integration. To the contrary, if a great time constant is selected, the action of following up the DC level change in the signal is rendered slow to consequently bring about a failure in attaining a satisfactory clamping operation.